Using longitudinal administrative data from a large elite research university, this paper analyzes the role of peers and grades in determining major persistence in the life and physical sciences. In the physical sciences, analyses using within-course, across-time variation show that ex-ante measures of peer quality in a student's introductory courses has a lasting impact on the probability of persisting in the major. This peer effect exhibits important non-linearities such that weak students benefit from exposure to stronger peers while strong students are not dragged down by weaker peers. In both the physical and the life sciences, I find evidence that students are "pulled away" by their high grades in non-science courses and "pushed out" by their low grades in their major field. In the physical sciences, females are found to be more responsive to grades than males, consistent with psychological theories of stereotype vulnerability. (Contains 7 tables and 10 figures.)